Cook chicken or use canned or pre-cooked chicken breast. Let cool, if necessary, and cut into bite-sized cubes. Cook puff pastries as directed while preparing the rest of the meal. In a large saucepan, heat frozen vegetables, milk, and cream of celery soup over medium heat until vegetables are warm. Add chicken, garlic powder, dried onion, and black pepper. Mix thoroughly and continue to cook until heated through. After pastries are removed from the oven, let cool 2-3 minutes. Then, remove the tops and fill with vegetable/chicken filling. Serve immediately. It is best to store any leftover pastries and filling separately to preserve the texture of the pastries.